Placental Macrophages Provide Information About Fetal Brain Microglia

Key findings

In mice, maternal obesity primed both placental CD11b+ macrophages and fetal brain microglia to overproduce a proinflammatory cytokine in response to immune challenge

Production of the proinflammatory cytokine by placental CD11b+ cells was strongly correlated with that of microglia, suggesting that fetal placental macrophages can provide information about the behavior of fetal brain microglia

In sex-stratified analyses, only male obesity-exposed brain and placental cells had significant reactivity to immune challenge, suggesting the male placenta and fetal brain may be more vulnerable to immune priming in the setting of maternal obesity

In the United States, one in three women is obese at the start of pregnancy. Maternal obesity increases the long-term risk of neurodevelopmental problems in the child, including autism spectrum disorder, cognitive deficits, attention-deficit/hyperactivity disorder (ADHD), anxiety, depression, disordered eating and cerebral palsy.

Obesity is a state of chronic low-grade metabolically induced inflammation. In a normal pregnancy, the placenta is relatively protected from inflammation because decidual and placental macrophages assume an anti-inflammatory state to enable immune tolerance of the fetus. In contrast, maternal obesity activates proinflammatory pathways in the placenta. Animal studies have demonstrated brain inflammation in the offspring of obese females, but little is known about whether placental inflammation correlates with fetal brain inflammation in the setting of maternal obesity.

A team led by Andrea G. Edlow, MD, a maternal-fetal medicine specialist and physician investigator in the Vincent Center for Reproductive Biology, and Staci Bilbo, PhD, director of research for the Lurie Center for Autism, had the insight that placental macrophages might provide information about the behavior of fetal brain immune cells because the two types of cells have a common origin in the fetal yolk sac. In the International Journal of Developmental Neuroscience, they report that maternal obesity primes both placental macrophages and fetal brain microglia to overproduce the proinflammatory cytokine tumor necrosis factor-alpha (TNF-α) in response to the immune challenge, and that placental macrophages might prove useful as a biomarker of aberrant fetal brain immune activation in maternal obesity.

Rationale for the Experiment

Dr. Edlow's group explains that, in rodents, yolk-sac–derived macrophages colonize the developing brain as early as embryonic day 9.5. In humans as well as rodents, microglia continue to proliferate during the first weeks after birth, forming a self-renewing pool that lasts throughout the lifespan.

Thus, adult microglia have embryonic origins, and fetal exposure to inflammation (e.g., from maternal obesity) might have lifelong neurodevelopmental consequences. However, microglia cannot be directly evaluated in a living fetus or newborn. The research team wondered whether placental macrophages could provide a window into fetal microglial function.

Methods

The researchers divided female mice into two groups. One group ate a high-fat diet for 8-10 weeks pre-breeding, and the other was fed a low-fat diet. Obesity was defined as ≥30% increase in prepregnancy weight compared to age-matched controls. All females were bred to male mice that had been fed the control diet.

On embryonic day 17.5, the pregnant mice were euthanized, and fetal brain microglia and placental macrophages were isolated using Percoll gradients followed by incubation with CD11b+ microbeads. CD11b is an integrin family member commonly used to identify microglia in the brain. Day 17.5 was selected because that timepoint permitted assessment of immune function prior to sex hormone production.

Purified CD11b+ and CD11b– cells from brains and placentas were treated with lipopolysaccharide (LPS) or neuro media alone. The researchers then measured the stimulated and unstimulated cells' production of TNF-α.

Responsiveness of Brain and Placental CD11b+ Cells to Immune Challenge

Baseline production of TNF-α by CD11b+ brain and placental cells was not affected by maternal obesity. However, the proinflammatory response to LPS was significantly increased by maternal obesity.

The researchers infer a "two hit" model. In isolation, maternal obesity may not change the behavior of fetal microglia and placental macrophages, but it primes the cells to overrespond to a second immune challenge. This finding may help explain why maternal obesity is a risk factor for neurodevelopmental morbidity, but not all offspring are affected.

Sex Differences in Responsiveness

In a prespecified sex-stratified analysis of obesity-exposed cells, brain and placental cells from male fetuses, but not from female fetuses, showed statistically significant increases in LPS reactivity (production of TNF-α) over controls.

Correlation of Placental Cell and Brain Cell Reactivity

In the setting of maternal obesity, the team noted a strong and significant correlation between the LPS reactivity of placental CD11b+ cells and the reactivity of microglia.

This analysis suggests to the researchers that the behavior of placental CD11b+ cells in response to immune challenge may be a reliable proxy for the behavior of brain microglia in the setting of maternal obesity.

The data will have broader implications for the study of fetal brain development, they add, if these placental macrophages and corresponding brain microglia can be further characterized in a variety of maternal exposure states.

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